Two years is a long time to be completely nonverbal, even for a second language learner.

From your description, I'm guessing the child has selective mutism. This is a type of anxiety disorder wherein the person talks in some comfortable settings (usually at home, or with certain people, or with stuffed animals/pets) but does not talk in other settings. The settings in which the person is mute are not necessarily scary places. It's just that for some reason, at some point, the person thought they were anxiety-provoking enough to stop talking in them. And once talking stops, it's hard to get it started again. Selective mutism is a relatively rare disorder. I've only ever worked with three kids with it. But in my opinion, it isn't all that much different from the reluctance to speak up that most of us have at some point--it's just much more pervasive.

To get more info on selective mutism, try www.ASHA.org (the American Speech-Language Hearing Association). I also recently heard of some sort of national association dealing just with selective mutism (sometimes also called elective mutism... although I don't like the term, because it implies the person is being mute on purpose, and could easily stop if he/she wanted to. That's not the case). I don't know that URL, but you could do a google search for it.

Treatment for selective mutism is usually conducted by psychotherapists, counselors, and other mental health professionals. Speech-language pathologists (like myself) are sometimes included as part of the team, but are rarely the case managers.

You asked about how to help this student. Here are my recommendations:
1) Contact the parents. Ask about when the student DOES talk, and who she talks with. If possible, it would also be useful to know whether the parents suspect any other communication disorder (imprecise pronunciation? Low vocab level? Immature grammar? Weird social interactions? Etc?), and also whether there is any family history of communication or anxiety disorders.
2) If, after talking with the parents, you suspect there is a real disorder here (very likely, I would think), then refer the child for a complete evaluation. If you are in the US then the child will probably be eligible for a free eval through the public schools. Contact your local school district's Child Study Team or Special Services Department.

And in the meantime:
1) Using more signs is okay. I'm not the one to teach these, but there are some websites that have them. Note that American Sign Language(ASL) is a language unto itself, whereas Signing Exact English (SEE) is a sign language developed to mirror English grammar. Therefore, SEE might be better if you want to eventually transition to verbal English, and ASL might be better if you want the student to learn an actual language that native "speakers" use to communicate with each other.
2) Try to decrease the speaking pressure. Don't force the student to talk. And if the child does communicate verbally, whatever you do--don't make a big deal out of it!!! Just smile normally, perhaps casually say good job, and move on to the next thing. You can celebrate later when not with the student. Celebrations will just freak her out.
3) Allow non-verbal communication. And, in really relaxed settings, model and casually encourage the student to mouth or whisper words. Often just opening the mouth and moving it silently, as though speaking, will lead eventually to whispering, and that will eventually lead to talking.
4) Don't expect rapid progress. One of my students slowly, slowly started whispering and then talking loud enough to barely hear. And she never got much louder than that. Another decided that when she changed schools the following year she was going to talk normally, and she did. It always takes time, and the disorder doesn't always go away completely.

Thank you so much for your response! I am first meeting with the speech teacher to determine if there is any physical reason for her not speaking. I am going to meet with her parents next.
I still cannot believe that she made it through all of last year with no verbal communication and no one thus far has done much of anything to check out why. I can see blaming the language barriers and a possible silent period for a while, but for over a year?
Her non-verbal communication is getting much better. I have taught her to knock on her desk when she needs help with something (as opposed to her simply staring at me and waiting for me to see her - which can be troublesome if I am working with another student in another part of the room!) She has picked up on the bathroom sign.
I will definitley look into the selective mutism disorder! I had never heard of this by that name so it is a definite possibilibty! Thanks again for the response!

One more thing though--I don't know about your school district, but in mine the parents must give permission before the speech teacher examines the child in any way.

You might want to check whether that's true where you live before having the speech teacher give an oral mechanism evaluation. You never know how parents are going to react to intervention for their kid... and with the relatively high possiblility of anxiety/social interaction disorders running in families... well, let me just say there's no point in making an enemy of the parents before you even get started.

Thank you! I had one of my friends from college who graduated in Deaf Studies/Interpretation send me a book of basic signs. It's helping pretty well.

It was found that she does have Selective Mutism. I am happy to report that she is lauging out loud in class now, and has even began (just this week) making sounds and sort of grunting to communicate (while still using the non-verbal communication she has been using). Im very excited about this. Im ready for the break-through!!